Field of the Invention
This invention generally relates to a heat exchanger adapted to be used in a vapor compression system. More specifically, this invention relates to a heat exchanger including a refrigerant distributor having a first tray part and a plurality of second tray parts.
Background Information
Vapor compression refrigeration has been the most commonly used method for air-conditioning of large buildings or the like. Conventional vapor compression refrigeration systems are typically provided with an evaporator, which is a heat exchanger that allows the refrigerant to evaporate from liquid to vapor while absorbing heat from liquid to be cooled passing through the evaporator. One type of evaporator includes a tube bundle having a plurality of horizontally extending heat transfer tubes through which the liquid to be cooled is circulated, and the tube bundle is housed inside a cylindrical shell. There are several known methods for evaporating the refrigerant in this type of evaporator. In a flooded evaporator, the shell is filled with liquid refrigerant and the heat transfer tubes are immersed in a pool of the liquid refrigerant so that the liquid refrigerant boils and/or evaporates as vapor. In a falling film evaporator, liquid refrigerant is deposited onto exterior surfaces of the heat transfer tubes from above so that a layer or a thin film of the liquid refrigerant is formed along the exterior surfaces of the heat transfer tubes. Heat from walls of the heat transfer tubes is transferred via convection and/or conduction through the liquid film to the vapor-liquid interface where part of the liquid refrigerant evaporates, and thus, heat is removed from the water flowing inside of the heat transfer tubes. The liquid refrigerant that does not evaporate falls vertically from the heat transfer tube at an upper position toward the heat transfer tube at a lower position by force of gravity. There is also a hybrid falling film evaporator, in which the liquid refrigerant is deposited on the exterior surfaces of some of the heat transfer tubes in the tube bundle and the other heat transfer tubes in the tube bundle are immersed in the liquid refrigerant that has been collected at the bottom portion of the shell.
Although the flooded evaporators exhibit high heat transfer performance, the flooded evaporators require a considerable amount of refrigerant because the heat transfer tubes are immersed in a pool of the liquid refrigerant. With recent development of new and high-cost refrigerant having a much lower global warming potential (such as R1234ze or R1234yf), it is desirable to reduce the refrigerant charge in the evaporator. The main advantage of the falling film evaporators is that the refrigerant charge can be reduced while ensuring good heat transfer performance. Therefore, the falling film evaporators have a significant potential to replace the flooded evaporators in large refrigeration systems.
In general, the rate of heat transfer between a surface (e.g., a surface of a heat transfer tube) and a substance (e.g., refrigerant) in a liquid state is much greater than the rate of heat transfer between the surface and the same substance in a gaseous state. Therefore, it is important for effective and efficient heat transfer performance to keep the tubes in the evaporator covered, or wetted, with liquid refrigerant during operation. With a flooded evaporator in which the tubes are immersed in a pool of the liquid refrigerant, performance of the evaporator can be maintained without significant degradation by controlling the liquid level within the evaporator shell even when the refrigerant circulation condition fluctuates. However, in a falling film evaporator, if all of refrigerant evaporates at an upper region of the tube bundle before it reaches a lower region, the lower tubes are left unwetted, thereby incapable of affecting heat transfer. Therefore, it is especially important in a falling film evaporator that there be a sufficient flow of liquid refrigerant over the tube bundle even when the refrigerant circulation condition fluctuates.
U.S. Patent Application Publication No. 2009/0178790 discloses a falling film evaporator including a refrigerant distribution assembly having an outer distributor and an inner distributor disposed within the outer distributor. Two-phase vapor-liquid refrigerant from a condenser first flows in the inner distributor. Vapor component of the two-phase refrigerant is discharged from the inner distributor into the outer distributor via a plurality of apertures formed in an upper portion of the inner distributor. A bottom portion of the inner distributor includes a plurality of openings through which the liquid component of the two-phase refrigerant is discharged into the outer distributor. The outer distributor has a plurality of apertures formed in lateral walls of the outer distributor to permit vapor refrigerant to flow from the outer distributor into a space within a hood enclosing the refrigerant distribution assembly. Liquid refrigerant collects in a bottom portion of the outer distributor and flows through distribution devices, such as nozzles, holes, openings, valves, etc., onto a tube bundle disposed below the refrigerant distribution assembly. Thus, with the refrigerant distribution assembly disclosed in this publication, vapor refrigerant is separated from liquid refrigerant, and only liquid refrigerant is discharged from the distribution devices toward the tube bundle.
U.S. Pat. No. 5,588,596 discloses a falling film evaporator including a vapor-liquid separator and a spray tree distribution system. The two-phase refrigerant from an expansion valve enters the vapor-liquid separator where the refrigerant is separated into vapor and liquid. The drain of the vapor-liquid separator is in fluid communication with and positioned above the spray tree distribution system which, in turn, is located above a tube bundle. The spray tree distribution system includes a manifold and a series of horizontal distribution tubes, each of which lies parallel to, in close proximity to, and directly above one uppermost tube of the tube bundle.